Final answer:
Einstein's theory of relativity is part of modern physics because it explains high-speed phenomena and strong gravitational fields, going beyond classical physics' constraints. The correct answer is b, pertaining to explaining phenomena at high speeds and gravitational fields.
Step-by-step explanation:
Einstein's theory of relativity is considered part of modern physics as opposed to classical physics because it explains phenomena at extremely high speeds and in strong gravitational fields, which were not adequately addressed by Newtonian mechanics. Unlike classical physics, the theory of relativity does not have constraints such as the limitation to objects moving much, much slower than the speed of light or being larger than the size of most molecules (about 10⁻⁹ m in diameter).
Modern physics encompasses both the theory of relativity and quantum mechanics, addressing the realms of the very fast and the very small, respectively, and offering a more comprehensive understanding of our universe. Option b, stating the theory of relativity explains phenomena at extremely high speeds and gravitational fields, is the correct answer to why it is considered part of modern physics.